fix: disable swap for executor_linux allocations

A comment in the nomad source code states that swapping for
executor_linux allocations is disabled but it wasn't.

Nomad wrote -1 to the memsw.limit_in_bytes cgroup file to disable
swapping.
This has the following problems:

1.) Writing -1 to the file does not disable swapping. It sets
    the limit for memory and swap to unlimited.
2.) On common Linux distributions like Ubuntu 16.04 LTS the
    memsw.limit_in_bytes cgroup file does not exist by default.
    The memsw.limit_in_bytes file only exist if the Linux kernel is
    build with CONFIG_MEMCG_SWAP=yes and either
    CONFIG_MEMCG_SWAP_ENABLED=yes or when the kernel parameter
    swapaccount=1 is passed during boot.
    Most Linux distributions disable swap accounting by default because
    of higher memory usage.
    Nomad silently ignores if writing to the memsw.limit_in_bytes file
    fails. The allocation succeeds, no message is logged to notify the
    user.

To ensure that disabling swap works on common Linux kernels, disable
swapping by writing 0 to the memory.swappiness file.
Using the memory.swappiness file only requires that the kernel is
compiled with CONFIG_MEMCG=yes. This is the default in common Linux
kernels.

client/driver/executor/executor_linux.go

@@ -83,7 +83,8 @@ func (e *UniversalExecutor) configureCgroups(resources *structs.Resources) error
 		// Total amount of memory allowed to consume
 		e.resConCtx.groups.Resources.Memory = int64(resources.MemoryMB * 1024 * 1024)
 		// Disable swap to avoid issues on the machine
-		e.resConCtx.groups.Resources.MemorySwap = int64(-1)
+		var memSwappiness int64 = 0
+		e.resConCtx.groups.Resources.MemorySwappiness = &memSwappiness
 	}
 
 	if resources.CPU < 2 {